In recent years, microcomputers have been applied to machines such as office automation machines. Microcomputers used in such machines are required to have low power consumption, because the machines are applied with a power supply from batteries such machines being of small size and light weight.
In order to achieve low power consumption, microcomputers are programmed to operate with low power consumption mode called a "stand-by mode". In the stand-by mode, the microcomputer halts its processing operation while maintaining information of internal states of the microcomputer in registers, flags, RAMs, etc. The microcomputer goes into the stand-by mode by carrying out a stand-by control command, and is released from the stand-by mode by receiving a system reset signal.
A conventional stand-by control circuit for controlling stand-by mode operation of a microcomputer includes a stand-by flag of a set/reset type flip-flop, a power ON detecting circuit which supplies a power ON detecting signal to a reset input terminal of the stand-by flag, an AND gate which has two input terminals supplied with a writing signal and a signal of a bus line, and a bus driver which is supplied with an output signal of the stand-by flag and controlled by a reading signal to supply a stand-by signal to the bus line. The power ON detecting circuit usually includes a transistor which is maintained to be in the ON state.
In operation, when the power supply voltage is applied to the microcomputer including the stand-by control circuit, the power ON detecting circuit detects the power ON state and the power ON detecting signal becomes active and is supplied to the stand-by flag to be reset. The stand-by flag is set when the AND gate supplies a set signal of logic level "1" on condition that the writing signal is active and the bus line is a high level. An output signal of the stand-by flag is supplied to the bus line through the bus driver when a reading signal supplied to the bus driver becomes active. The power ON detecting signal supplied from the power ON, detecting circuit becomes active, just after the power supply is turned on and remains active until the power supply voltage becomes as high as the inversion level. Then, the power ON detecting signal becomes inactive. In such operation, a constant current flows through the transistor which is maintained at an ON state in the power ON detecting circuit.
In this conventional stand-by control circuit, the stand-by flag is controlled to be at a set state, when a system is reset after the release of the stand-by mode, while the stand-by flag is controlled to be at a reset state, when the system is reset by turning a power supply on, so that the two reset states of the system are easily distinguished.
According to the conventional stand-by control circuit, however, there is a disadvantage in that it is difficult to sort out microcomputers having current leakage of several .mu.A which are thereby determined to be defective in testing microcomputers, because a constant current of several .mu.A flows constantly through the power ON detecting circuit. The current leakage of several .mu.A is included at the normal temperature by junction current leakage, sub-threshold current leakage, etc.. In order to overcome such a disadvantage, testing is carried out under a high temperature to increase the leakage currents to the level as large as some tens of .mu.A. In this testing method, however, there is another disadvantage in that process of the testing becomes complicated, so that an advantage of LSIs of low cost may not be realized.